go/types: better handle arrays whose length expression is invalid

While doing that, establish a negative value as signal for unknown
array lengths and adjust various array-length processing code to
handle that case.

Fixes #23712.

Change-Id: Icf488faaf972638b42b22d4b4607d1c512c8fc2c
Reviewed-on: https://go-review.googlesource.com/93438
Reviewed-by: Alan Donovan <adonovan@google.com>

6 files changed, 38 insertions, 7 deletions

diff --git a/src/go/types/builtins.go b/src/go/types/builtins.go
index f22851e240..785daec331 100644
--- a/src/go/types/builtins.go
+++ b/src/go/types/builtins.go
@@ -158,7 +158,11 @@ func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 			// function calls; in this case s is not evaluated."
 			if !check.hasCallOrRecv {
 				mode = constant_
-				val = constant.MakeInt64(t.len)
+				if t.len >= 0 {
+					val = constant.MakeInt64(t.len)
+				} else {
+					val = constant.MakeUnknown()
+				}
 			}
 
 		case *Slice, *Chan:
diff --git a/src/go/types/predicates.go b/src/go/types/predicates.go
index 3aa4878cce..1ca146f590 100644
--- a/src/go/types/predicates.go
+++ b/src/go/types/predicates.go
@@ -150,7 +150,9 @@ func identical(x, y Type, cmpTags bool, p *ifacePair) bool {
 		// Two array types are identical if they have identical element types
 		// and the same array length.
 		if y, ok := y.(*Array); ok {
-			return x.len == y.len && identical(x.elem, y.elem, cmpTags, p)
+			// If one or both array lengths are unknown (< 0) due to some error,
+			// assume they are the same to avoid spurious follow-on errors.
+			return (x.len < 0 || y.len < 0 || x.len == y.len) && identical(x.elem, y.elem, cmpTags, p)
 		}
 
 	case *Slice:
diff --git a/src/go/types/sizes.go b/src/go/types/sizes.go
index 0821a61359..eb274799f4 100644
--- a/src/go/types/sizes.go
+++ b/src/go/types/sizes.go
@@ -132,9 +132,10 @@ func (s *StdSizes) Sizeof(T Type) int64 {
 		}
 	case *Array:
 		n := t.len
-		if n == 0 {
+		if n <= 0 {
 			return 0
 		}
+		// n > 0
 		a := s.Alignof(t.elem)
 		z := s.Sizeof(t.elem)
 		return align(z, a)*(n-1) + z
diff --git a/src/go/types/testdata/importC.src b/src/go/types/testdata/importC.src
index f50f7f33d3..f55be2d5c5 100644
--- a/src/go/types/testdata/importC.src
+++ b/src/go/types/testdata/importC.src
@@ -20,7 +20,7 @@ type T struct {
 	Ordinal int
 }
 
-func f(args []T) {
+func _(args []T) {
 	var s string
 	for i, v := range args {
 		cname := C.CString(v.Name)
@@ -33,3 +33,22 @@ type CType C.Type
 
 const _ CType = C.X // no error due to invalid constant type
 const _ = C.X
+
+// Test cases extracted from issue #23712.
+
+func _() {
+	var a [C.ArrayLength]byte
+	_ = a[0] // no index out of bounds error here
+}
+
+// Additional tests to verify fix for #23712.
+
+func _() {
+	var a [C.ArrayLength1]byte
+	_ = 1 / len(a) // no division by zero error here and below
+	_ = 1 / cap(a)
+	_ = uint(unsafe.Sizeof(a)) // must not be negative
+
+	var b [C.ArrayLength2]byte
+	a = b // should be valid
+}
diff --git a/src/go/types/type.go b/src/go/types/type.go
index 50e3c6e4d0..afdbb680f8 100644
--- a/src/go/types/type.go
+++ b/src/go/types/type.go
@@ -97,9 +97,11 @@ type Array struct {
 }
 
 // NewArray returns a new array type for the given element type and length.
+// A negative length indicates an unknown length.
 func NewArray(elem Type, len int64) *Array { return &Array{len, elem} }
 
 // Len returns the length of array a.
+// A negative result indicates an unknown length.
 func (a *Array) Len() int64 { return a.len }
 
 // Elem returns element type of array a.
diff --git a/src/go/types/typexpr.go b/src/go/types/typexpr.go
index aedd71e918..e86834efdd 100644
--- a/src/go/types/typexpr.go
+++ b/src/go/types/typexpr.go
@@ -380,6 +380,9 @@ func (check *Checker) typOrNil(e ast.Expr) Type {
 	return Typ[Invalid]
 }
 
+// arrayLength type-checks the array length expression e
+// and returns the constant length >= 0, or a value < 0
+// to indicate an error (and thus an unknown length).
 func (check *Checker) arrayLength(e ast.Expr) int64 {
 	var x operand
 	check.expr(&x, e)
@@ -387,7 +390,7 @@ func (check *Checker) arrayLength(e ast.Expr) int64 {
 		if x.mode != invalid {
 			check.errorf(x.pos(), "array length %s must be constant", &x)
 		}
-		return 0
+		return -1
 	}
 	if isUntyped(x.typ) || isInteger(x.typ) {
 		if val := constant.ToInt(x.val); val.Kind() == constant.Int {
@@ -396,12 +399,12 @@ func (check *Checker) arrayLength(e ast.Expr) int64 {
 					return n
 				}
 				check.errorf(x.pos(), "invalid array length %s", &x)
-				return 0
+				return -1
 			}
 		}
 	}
 	check.errorf(x.pos(), "array length %s must be integer", &x)
-	return 0
+	return -1
 }
 
 func (check *Checker) collectParams(scope *Scope, list *ast.FieldList, variadicOk bool) (params []*Var, variadic bool) {